Turn off the circuit cleorotlnd by the switch, at the service panel. If not sure which one, turn on light with the switch, and turn off switches in service panel until the light goes off. Turn back on the other service panel switches for other circuits! From the switch, locate the neutral wires. They SHOULD be the ones not connected to the switch, white, and hopefully “pigtailed” together. IF YOU DON’T FIND THAT, YOU CANNOT USE THE SWITCH TO TAP INTO! But otherwise, you won’t have to worry about the switch being on or off. You will pigtail your white into that pigtail. You will pigtail your black into the HOT side of the switch. The only way to determine that is by turning the service panel switch back on with any type of circuit tester bridging the gap with one of the black wires off the light switch. If the circuit registers hot, take that wire and pigtail it for your black. Also be sure to pigtail the bare ground wires. On the other side of the wall, where you want your GFCI, cut a small hole to stick your finger in to see where the stud might be. Your light switch will give you the best clue, as that box will likely be on a stud. You will use an “old work” box to house the GFCI. It has two clips that screw it tight against the sheetrock. Once you have your hole for the box, drop your new wire down from the switch, and out the new hole, giving about 8 inches of slack. Put through one of the holes in the box, and strip the outer cover off the Romex. Install the box into the sheetrock hole. You will want to use 12 or 14 gauge Romex (also known as NM) wire cable. On the ends of the new wire, connect the black to the gold colored “LINE” screw, making sure to curl around the screw as much as possible, with very little extra wire (strip about 1/2 inch). The white will go to the silver “LINE” screw. The “LOAD” is only for devices you want to be GFCI protected, and hardwired.